It has been proposed that a cell can express genes that were hitherto inactive only after cell division, suggesting a causal association between DNA synthesis and differentiation. The validity of this hypothesis is being tested in the chitogenous cells of Galleria, Tenebrio and Drosophila. Integument from day-1 penultimate instar larvae, on implantation into a mid last instar larva deposits a pupal cuticle. However on implantation into a late last instar larva, the epidermal cells secrete a larval cuticle. DNA synthesis in the epidermal cells of the implants, programmed to secrete a larval or pupal cuticle, was monitored by an autoradiographic analysis of H3-thymidine incorporation. The data thus far obtained show that larval epidermal cells do not have to engage in DNA synthesis prior to expression of pupal genes. We are currently monitoring DNA synthesis using other nucleosides as well, since the choice of nucleosides may influence the results. Similar studies of implants during pupal-adult transformation are in progress and will be extended to Tenebrio. The temperature sensitive homeotic mutants in Drosophila are suitable to test the hypothesis and the mutant Ophthalmoptera is being studied. BIBLIOGRAPHIC REFERENCES: Reprogramming in the absence of DNA synthesis in Galleria larval epidermis. William H. Kastern and A. Krishna Kumaran. Cell Differentiation 4 (1975) 45-53.